1. Field of the Invention
The present invention relates to switch modules that transmit and receive a plurality of communication signals using a common antenna.
2. Description of the Related Art
In recent years, due to multi-band communication in cellular phones and the like, communication apparatuses have reached a level where they can transmit and receive a plurality of signals having different frequencies using a common antenna. Hence, communication apparatuses have increasingly used a switch module that connects a plurality of communication circuits to a common antenna through switching (refer to, for example, Japanese Unexamined Patent Application Publication No. 2008-271420).
FIG. 5A is a block diagram illustrating an example of a general circuit configuration of such a switch module.
Referring to FIG. 5A, a front end circuit FEC includes a switch circuit SW, a common-port-side circuit 104, and switching-port-side circuits 107A to 107H. As external connection terminals, the switch circuit SW also includes an antenna terminal ANT, a power supply terminal Vdd, control terminals Vc1 to Vc4, a low-frequency-side transmission terminal LTx, a high-frequency-side transmission signal terminal HTx, transmission/reception signal terminals TRx1 to TRx6, and a ground terminal GND.
The switch circuit SW includes a common port PIC01 and switching ports PIC11 to PIC18, and is configured to be capable of switching among the switching ports PIC11 to PIC18 so that the switching ports PIC11 to PIC18 may be connected to the common port PIC01. The common-port-side circuit 104 is connected between the antenna terminal ANT and the common port PIC01 of the switch circuit SW. The switching-port-side circuits 107A to 107H are respectively connected between the switching ports PIC11 to PIC18 and the low-frequency-side transmission terminal LTx, the high-frequency-side transmission signal terminal HTx, and the transmission/reception signal terminals TRx1 to TRx6.
In the front end circuit FEC, the common-port-side circuit 104 includes a capacitor which is connected to the antenna as a shunt, a first inductor which is connected in series with the antenna, and a second inductor which is connected to the first inductor as a shunt. The common-port-side circuit 104 is formed as an electrostatic damage protection circuit to prevent intrusion of static electricity from the antenna into the common port PIC01 of the switch circuit SW.
The switching-port-side circuit 107A, which is connected to the low-frequency-side transmission terminal LTx, is formed as a low pass filter that removes the harmonic components of a low-frequency-side transmission signal. The switching-port-side circuit 107B, which is connected to the high-frequency-side transmission signal terminal HTx, is formed as a low pass filter that removes the harmonic components of a high-frequency-side transmission signal.
The front end circuit FEC described above is usually formed as a switch module using a multilayer substrate. The circuit elements of the switch circuit SW, the circuit elements of the common-port-side circuit 104, the circuit elements of the switching-port-side circuits 107A to 107H, and the like are formed using electrode patterns which are formed on the top surface and bottom surface of the multilayer substrate and inside the multilayer substrate and using surface mount components which are surface mounted on the multilayer substrate.
FIG. 5B is a plan view illustrating, using dotted lines, the major portions of electrode patterns within the multilayer substrate of a switching module according to an existing configuration when the multilayer substrate is viewed from the mounting surface side thereof.
A multilayer substrate 111 illustrated in FIG. 5B, which defines the front end circuit FEC described above, includes external connection terminals provided on the mounting surface with which the multilayer substrate 111 is to be mounted on an external substrate. The multilayer substrate 111 includes a plurality of via electrodes (not illustrated) and pattern electrodes provided within the substrate.
In the multilayer substrate 111, the antenna terminal ANT and the high-frequency-side transmission signal terminal HTx are arranged next to each other with the ground terminal GND therebetween. A pattern electrode 112 that is connected to the antenna terminal ANT and forms part of a capacitor is arranged close to an electrode pattern 113 that is connected to the high-frequency-side transmission signal terminal HTx and forms a lead wiring line.
In recent years, external connection terminal patterns have become very fine due to a reduction in the size of switching modules and, hence, the antenna terminal ANT and the high-frequency-side transmission signal terminal HTx are arranged close to each other in an increasing number of cases. As a result, there have been cases in which the circuit elements and lead wiring line of the common-port-side circuit are arranged close to, and are coupled through an electromagnetic field to, the lead wiring lines of the switching-port-side circuits, whereby the isolation characteristics of the switch module are degraded.
In particular, when the capacitor of the common-port-side circuit is capacitively coupled to the lead wiring lines of the low pass filters of the switching-port-side circuits, attenuation in the attenuation bands of the low pass filters becomes small, whereby the amounts of removed harmonic components become small.